Buildings are usually designed with a space between a drop ceiling and a structural floor from which the ceiling is suspended to serve as a return air plenum for elements of heating and cooling systems as well as serving as a convenient location for the installation of communications cables and other equipment, such as power cables. Alternatively, the building can employ raised floors used for cable routing and plenum space. Communications cables generally include voice communications, data and other types of signals for use in telephone, computer, control, alarm, and related systems, and it is not uncommon for these plenums and the cables therein to be continuous throughout the length and width of each floor, which can introduce safety hazards, both to the cables and the buildings.
When a fire occurs in an area between a floor and a drop ceiling, it may be contained by walls and other building elements which enclose that area. However, if and when the fire reaches the plenum space, and especially if flammable material occupies the plenum, the fire can spread quickly throughout the entire floor of the building. The fire could travel along the length of cables which are installed in the plenum if the cables are not rated for plenum use, i.e., do not possess the requisite flame and smoke retardation characteristics. Also, smoke can be conveyed through the plenum to adjacent areas and to other floors with the possibility of smoke permeation throughout the entire building.
As the temperature in a non-plenum rated jacketed cable rises, charring of the jacket material begins. Afterwards, conductor insulation inside the jacket begins to decompose and char. If the charred jacket retains its integrity, it still functions to insulate the core; if not, however, it ruptures due either to expanding insulation char or to pressure of gases generated from the insulation, and as a consequence, exposes the virgin interior of the jacket and insulation to the flame and/or the elevated temperatures. The jacket and the insulation begin to pyrolize and emit more flammable gases. These gases ignite and, because of air drafts in the plenum, burn beyond the area of flame impingement, thereby propagating flame and generating smoke and toxic and corrosive gases.
Because of the possibility of flame spread and smoke evolution, as a general rule, the National Electrical Code (NEC) requires that power-limited cables in plenums be enclosed in metal conduits. However, the NEC permits certain exceptions to this requirement. For example, cables without metal conduits are permitted, provided that such cables are tested and approved by an independent testing agent, such as Underwriters Laboratories (UL), as having suitably low flame spread and smoke generating or producing characteristics. The flame spread and smoke production of cables are measured using the UL 910, also known as the “Steiner Tunnel,” standard test method or, more recently, the NFPA 262 flame test for fire and smoke retardation characteristics of electrical and optical fiber cables used in air handling spaces, i.e., plenums.
Communication systems in the present day environment are of vital importance, and, as technology continues to become more sophisticated, such systems are required to transmit signals substantially error free at higher and higher bit rates. More particularly, it has become necessary to transmit data signals over considerable distances at high bit rates, such as megabits or gigabits per second, and to have substantially error free transmission. Thus, desirably, the medium over which these signals are transmitted must be capable of handling not only low frequency and voice signals, for example, but higher frequency data and video signals.
The most common cable in data communication involves an electrical conductor that is surrounded by a single layer of insulation. Plenum applications typically require that the insulation used contain polymers, generally fluoropolymers. Low melt flow index (MFI) fluoropolymers more readily accommodate the flame and smoke performance when tested per UL 910. The low MFI polymers, however, typically have a higher electrical dissipation factor which can cause poor electrical performance typically at frequencies higher than 100 MHz.
Currently most plenum cables use a higher melt flow material as insulation. That configuration sacrifices the flame and smoke characteristics by using higher melt flow materials to obtain the desired electrical characteristics. Those insulations typically pass the UL910 test, but require the addition of polyvinylchloride (PVC) jacket sheaths or other costly components. For specific high-end 10G applications, the cable requires flame characteristics higher than what could be obtained by exterior PVC jacketing.
It is apparent from the foregoing discussion that there remains a need for a flame retardant, and low-smoke generating communication cable that does not sacrifice transmission properties for fire and smoke resistance.